Low loss fuel cell configuration

ABSTRACT

Switches ( 306 ) are used to connect stacks of fuel cells ( 302, 304 ) in series or in parallel depending on the load to keep total cell voltage below a maximum input voltage requirement. The cells ( 302, 304 ) are coupled in parallel under low power conditions, and the cells are coupled in series under high power conditions to provide a more efficient system ( 500 ).

FIELD OF THE INVENTION

[0001] This invention relates generally to fuel cells and morespecifically to the efficiency of fuel cell systems.

BACKGROUND OF THE INVENTION

[0002] Fuel cells provide clean, direct current (DC) electricity. Fuelcells convert reactants, namely fuel and oxidant (air or oxygen), togenerate electric power and reaction products. A typical fuel cell powersource can be constructed from a stack of cells coupled in series asshown in FIG. 1. Fuel cells exhibit a substantially linear decreasingoutput voltage as the power is increased as represented by FIG. 2 graph200.

[0003] A typical direct methanol fuel cell (DMFC) has an open cellvoltage of approximately 0.7 volts, with the cell voltage dropping toapproximately 0.25 volts near peak power. The operating voltage forportable devices is typically in the range of 1.5-15 volts. For example,a two-way radio battery may require 10 volts to charge properly. A DC/DCboost converter can be used to provide a 10 V output to such devices.The total cell voltage going to the input of the regulator cannot exceedthe output voltage of the regulator. The total number of cells isdetermined by calculating the regulator output by the open cell voltage(in this example 10V/0.7V would be 14 cells). When the system operatesat near peak power, the cell voltage is thus 14×0.25 volts=3.5 volts. Itis known that the efficiency of the regulator is dependent on the inputvoltage to the converter. Generally, closer input voltage to outputvoltage provides higher efficiency. As an example, some DC/DC convertersare rated to have 87% efficiency for converting from 5 volts to 10 voltsbut only 67% from 2 volts. A fuel cell configuration that increasesoverall system efficiency is highly desirable. For example, a fuel cellstructure that has an open cell voltage of 10 volts that is capable ofproviding a higher operating voltage near peak power would increaseoverall efficiency.

[0004] Accordingly, it would be highly desirable to have a fuel cellconfiguration that provides improved efficiency at higher power levels.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The present invention is illustrated by way of example and notlimitation in the accompanying figures, in which like referencesindicate similar elements, and in which:

[0006]FIG. 1 is a prior art block diagram representation of a stack offuel cells coupled in series;

[0007]FIG. 2 is a prior art graph of cell voltage versus power for atypical fuel cell;

[0008]FIG. 3 is a block diagram of a fuel cell structure having improvedefficiency in accordance with the present invention;

[0009]FIG. 4 is an example of a graph depicting cell voltage versuspower for the fuel cell structure of FIG. 3 in accordance with thepresent invention; and

[0010]FIG. 5 is a block diagram of a fuel cell system including theimproved fuel cell configuration formed in accordance with the presentinvention.

[0011] Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale.

DETAILED DESCRIPTION OF THE DRAWINGS

[0012] In accordance with the present invention, there is providedherein a fuel cell configuration in which stacks of cells are coupledeither in parallel or in series to maintain the total cell voltage belowa maximum input for a regulator while providing improved output power athigher output voltage levels.

[0013]FIG. 3 shows a fuel cell configuration 300 formed in accordancewith the present invention. In accordance with the present invention,fuel cell configuration 300 includes a first stack of fuel cellsconnected in series 302, a second stack of fuel cells connected inseries 304 and a plurality of switches 306 for coupling the second stackof fuel cells in parallel with the first stack of fuel cells under lowpower conditions and for coupling the first stack of fuel cells inseries with the second stack of fuel cells under high power conditions.Fuel cell configuration 300 shows the cells connected in series. In thisparticular example, each stack is shown having fourteen cells. Greateror fuel cells may be used depending on the application.

[0014]FIG. 4 is a graph 400 depicting an example of cell voltage 402versus power 404 in accordance with the cell configuration of FIG. 3.Designator 406 represents the operating condition in which the twostacks of cells are coupled in parallel and shows how the overall cellvoltage falls as the power being drawn is increased. Once the overallcell voltages reaches a predetermined level for a given load, the nextoperating condition is switched in and the first and second stacks 302,304 of cells become coupled in series as shown by designator 408. Cellvoltage is thus able to increase back up (due to the series connection)while the power drawn continues to increase.

[0015]FIG. 5 is a block diagram of a fuel cell system 500 including thefuel cell configuration formed in accordance with the present invention.First and second fuel cell stacks are operatively coupled to provide foreither parallel or series coupling in accordance with the presentinvention. The configuration in system 500 is a parallel configuration.The system further includes a controller 502, a DC/DC converter 504, aswitch 506, and a load 508. The switch 506 is preferably a switching ICunder control of controller 502. Switching IC 506 connects the stacks inseries or in parallel depending on the load to keep total cell voltagebelow the maximum input voltage requirement of the DC/DC regulator 504.

[0016] Using the 10-volt output as an example, instead of using onestack of fourteen cells, fuel cell configuration 300 provides two stacksof fourteen cells. Within each set or stack, all fourteen cells areconnected in series. At low power, the two stacks are connected inparallel. As more power is drawn from the cell, the voltage across thestack drops. When the voltage drops below 5 volts, the switching IC 506connects the two stacks in series. At near peak power, the cell voltagemeasures about 7 volts, at which point the efficiency is approximately10-15% higher than at 3.5 volts.

[0017] The number of cells in each stack is determined in accordancewith the voltage requirements of the application. Load, input voltagelimits, and cell characteristics all play a role in determining whatswitchover limits to set. Thus, while the examples described have shownwith two stacks of fourteen cells, the fuel cell configuration of thepresent invention could consist of a plurality of stacks with more orfewer cells within each stack.

[0018] The following steps summarize a method of deriving improved fuelcell efficiency through a fuel cell configuration formed in accordancewith the present invention. Initially, stacks of series-connected fuelcells are provided and switchably coupled in a parallel. The voltage ofthe parallel-coupled stacks is monitored and compared to a predeterminedlevel. In response to the voltage falling below the predetermined level,the stacks of fuel cells are switched into a series configuration, thusproviding a higher voltage and greater efficiency.

[0019] Accordingly, there has been provided an improved fuel cellconfiguration that provides greater efficiency to fuel cell systems. Avariety of products including but not limited to communicationsproducts, such as two-way radios and cell phones, can benefit from theoverall performance improvement with improved talk time performance.

[0020] In the foregoing specification, the invention has been describedwith reference to specific embodiments. However, one of ordinary skillin the art appreciates that various modifications and changes can bemade without departing from the scope of the present invention as setforth in the claims below. Accordingly, the specification and figuresare to be regarded in an illustrative rather than a restrictive sense,and all such modifications are intended to be included within the scopeof present invention.

[0021] Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeatures or elements of any or all the claims. As used herein, the terms“comprises,” “comprising,” or any other variation thereof, are intendedto cover a non-exclusive inclusion, such that a process, method,article, or apparatus that comprises a list of elements does not includeonly those elements but may include other elements not expressly listedor inherent to such process, method, article, or apparatus.

We claim:
 1. A fuel cell structure, comprising: a first stack of fuelcells connected in series; a second stack of fuel cells connected inseries; and a switch for coupling the second stack of fuel cells inparallel with the first stack of fuel cells under low power conditionsand for coupling the first stack of fuel cells in series with the secondstack of fuel cells under high power conditions.
 2. The fuel cell systemof claim 1, wherein the switch is activated when the cell voltage dropsbelow a predetermined level.
 3. A method of protecting a fuel cellsystem, comprising: providing first stack of fuel cells connected inseries; providing a second stack of fuel cells connected in series andswitchably coupled in parallel to the first stack; monitoring a voltageof the parallel-coupled stacks; comparing the voltage to a predeterminedlevel; and switching the second stack of fuel cells in series with thefirst stack of fuel cells in response to the voltage falling below thepredetermined level.
 4. A fuel cell system, comprising: a first stack offuel cells connected in series; a second stack of fuel cells connectedin series; and a plurality of switches for coupling the first and secondstacks in parallel for a first set of predetermined conditions and forconnecting the first and second stacks of cells in series for a secondset of predetermined conditions.
 5. The fuel cell system of claim 4,wherein each stack comprises fourteen fuel cells.
 6. A fuel cell system,including: a plurality of stacks, each stack comprising a plurality offuel cells coupled in series, the plurality stacks being switchablycoupled in parallel under a first set of operating conditions and eachstack being switchably coupled in series under a second set of operatingconditions; and a load drawing power from the parallel-coupled stacksunder the first set of conditions and then load drawing power from theseries coupled stacks under the second set of operating conditions. 7.The fuel cell system of claim 6, wherein the operating conditionsincludes a predetermined voltage level.
 8. The fuel cell system of claim6, wherein a power level is maintained above a predetermined power levelby switching from parallel coupling of the plurality of stacks to seriescoupling of the plurality of stacks.
 9. A fuel cell system, comprising:a first stack of cells; a second stack of cells; a voltage regulatoroperatively coupled to the first and second stacks, the voltageregulator having a maximum input voltage requirement; and a switching ICfor connecting the first and second stacks in series or in parallel suchthat the total cell voltage remains below the maximum input voltagerequirement of the regulator.
 10. The fuel cell system of claim 9,wherein the voltage regulator comprises a DC/DC converter.